1. Field of the Invention
This invention relates to writing instruments and, more specifically, to a housing assembly for a writing instrument formed by mechanically uniting a ferrule member and a barrel member to provide an internal chamber for writing fluid.
2. Description of the Prior Art
Modern writing instruments of the type comprising a hollow barrel which holds a reservoir for writing fluid and a nib projecting through a ferrule for transporting fluid from the reservoir to a writing surface are well known. The nib is generally mounted in and projects from the ferrule which is attached to the barrel holding the fluid reservoir for the writing instrument. Such writing instruments are preferably made of inexpensive material, such as polypropylene, nylon or other plastic materials, so that the instruments are disposable and are discarded upon complete use of the writing fluid therein contained.
Conventionally, the barrel and ferrule of such writing instruments have been assembled by welding, by chemically bonding, by a press fit operation, or by screwing the ferrule to the barrel to enclose a previously inserted fluid reservoir in the capsular internal chamber. The ferrule generally has an end portion which is inserted into an open end of the barrel.
One popular method of forming such a housing assembly from a barrel and ferrule which is used in automatic production is known as spin welding. In the spin welding technique, the adjacent complementary portions of the ferrule and the barrel are each of mating circular cross section and of a size to be inserted one within the other.
To this end, the barrels are normally advanced in a row side-by-side on an assembly line. A fluid reservoir is inserted into each barrel and filled with ink (if not prefilled). The ferrule is then positioned in alignment with the barrel, and the end portion of the ferrule is inserted into the adjacent end portion of the barrel so that the surfaces of the end portions contact one another. The barrel or the ferrule is then rotated relative to the other at high speed. The frictional contact between the surfaces of the two end portions melts and fuses the plastic material of the adjacent surfaces of the ferrule and the barrel to unite the parts into a housing assembly for a writing instrument. This procedure may be effected by gripping and spinning the ferrule while the barrel is held stationary and in alignment with the ferrule.
After the spin welding operation, a nib is inserted through the bore of the nib holder at the exposed end of the ferrule and into contact with the fluid reservoir positioned within the housing assembly.
Although carefully supervised spin welding techniques have been satisfactory for production, they incorporate certain disadvantages. The ferrule and the barrel must be composed of the same, or at least compatible, plastic materials to assure that a contiguous, permanent and leak-proof weld is obtained in the contiguous surfaces of the parts. Generally, the plastic materials must be identical. However, different plastic materials are sometimes necessary for use with different types of inks. Each ferrule and barrel of the differing plastic, being identically formed, are sometimes indeterminable as to composition. If an inadvertent attempt is made to weld a ferrule of one plastic material to a barrel of a different plastic material, the fusing of the contiguous surfaces may be incomplete or wholly defective. This flaw is not necessarily apparent from visual inspection. Thus, for instance, in removing the cap from a finished instrument, the ferrule may become disengaged from the barrel resulting in leakage or spillage of the ink from the fluid reservoir upon the person using the writing instrument.
Even with plastic materials which are compatible for the spin welding technique, this same type of flaw may occur if the surface tolerances between the parts is too great. Again, due to incomplete fusing of the contiguous surfaces of the ferrule and the barrel, evaporation of the volatile solvent in the marking fluid may occur, the parts may become disengaged, or leakage of the writing fluid may result.
Ultrasonic welding procedures suffer from the same infirmities found in the spin welding technique. Chemical bonding or adhesive methods for joining the components are time consuming and uneconomical, in addition to having the problems incurred in maintaining proper alignment in the assembly operation. An adhesive must be compatible with differing materials if inadvertent mixing of different plastic components occurs, or if differing plastics are intentionally used. Application of adhesive material is difficult, and an excess of adhesive material applied can result in its oozing around the exterior of the connecting joint, making an unsightly, and possibly unsatisfactory, writing instrument housing.
In the techniques of spin welding, of electric ultrasonic welding and of chemical bonding, the equipment generally used, because of its complexity, requires considerable maintenance which often results in interruptions in the automated assembly operation. Such interruptions for maintenance causes production losses and increased costs.